An ignition coil device for an internal combustion engine is inserted in a plug hole formed in a cylinder head and is connected to an ignition plug. When the ignition coil device is inserted in the plug hole, or when air in the plug hole is thermally expanded with operation of the internal combustion engine, the air has to be discharged from the inside of the plug hole to the outside.
For that purpose, an air bleeding hole (groove) for enabling the inside and the outside of the plug hole to communicate with each other is formed in the ignition coil device. However, it is also required to prevent water from entering the plug hole from the outside through the air bleeding hole.
In view of the above requirement, according to a technique disclosed in Patent Document 1, water is prevented from entering the plug hole through an air vent (air bleeding hole) as follows. A first groove communicating with the outside is formed on the lower-pressure terminal socket side in an area where a seal rubber for sealing an opening of the plug hole is mounted. Further, a second groove communicating with the mounted area of the seal rubber is formed, and a third groove communicating with the second groove is formed on the lower-pressure socket side. The first groove and the third groove are communicated with each other through an enclosed space.
When water is going to enter the plug hole from the outside, the water is stored in the space and is prevented from entering the inside of the plug hole. When air is discharged from the inside of the plug hole to the outside, the water stored in the space is discharged together to the outside.
Further, as disclosed in Patent Document 2, it is also known to install filtering means in a ventilation path 41.    Patent Document 1: JP, A 2000-291523    Patent Document 2: JP, A 2000-87837